Nonlinear optics in the extreme ultraviolet
ABSTRACT Nonlinear responses to an optical field are universal in nature but have been difficult to observe in the extreme ultraviolet (XUV) and soft X-ray regions owing to a lack of coherent intense light sources. High harmonic generation is a well-known nonlinear optical phenomenon and is now drawing much attention in attosecond pulse generation. For the application of high harmonics to nonlinear optics in the XUV and soft X-ray regime, optical pulses should have both large pulse energy and short pulse duration to achieve a high optical electric field. Here we show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically. The XUV pulses with pulse durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon above-threshold ionization of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of XUV pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics at shorter wavelengths.
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- "uld be an attosecond pulse train . The race was on to test this spectral phase relation . As attosecond pulses typically come with low pulse energies , nonlinear - optical autocorrelation commonly applied for femtosecond pulses was not immediately possible , and still is not a viable option for all photon - energy ranges ( Tzallas et al . , 2003 ; Sekikawa et al . , 2004 ) . Instead , the solution was a temporal cross correlation of the HHG light with a moderately intense and coherently locked copy of the 800 nm HHG driver pulse in a gas medium while observing photo - electron emission ( Paul et al . , 2001 ) . In this process , the odd harmonic photons with energy E 2n+1 ionized an electron with excess"
ABSTRACT: In the past two decades high-harmonic generation (HHG) has become a key process in ultra-fast science due to the extremely short time-structure of the underlying electron dynamics being imprinted in the emitted harmonic light bursts. After discussing the fundamental physical picture of HHG including continuum--continuum transitions, we describe the experimental progress rendering HHG to the unique source of attosecond pulses. The development of bright photon sources with zeptosecond pulse duration and keV photon energy is underway. In this article we describe several approaches pointed toward this aim and beyond. As the main barriers for multi-keV HHG, phase-matching and relativistic drift are discussed. Routes to overcome these problems are pointed out as well as schemes to control the HHG process via alterations of the driving fields. Finally, we report on how the investigation of fundamental physical processes benefits from the continuous development of HHG sources.Advances in Atomic, Molecular, and Optical Physics 01/2012; 61. DOI:10.1016/B978-0-12-396482-3.00004-1 · 3.40 Impact Factor
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ABSTRACT: High-order harmonic emission can be confined to the leading edge of an 800 nm driver laser pulse under moderately intense focusing conditions (7×10^14 W/cm^2) (Pfeifer et al. in Opt. Express 15:17120, 2007). Here, the experimentally observed curtailment of harmonic production on the leading edge of the driver pulse is shown to be controlled by an ionization-induced phase-matching condition. The transient plasma density inherent to the process of high-harmonic generation terminates the harmonic emission by an ultrafast loss of phase matching on the leading edge of the laser pulse. The analysis is supported by a reconstruction of the in situ intensity envelope of the driver pulse with attosecond temporal resolution, performed by measurements of the carrier-envelope phase dependence of individual half-cycle harmonic cutoffs. The method opens the way to wavelength-tunable isolated attosecond pulse generation.Applied Physics B 11/2008; 93(2-3). DOI:10.1007/s00340-008-3187-z · 1.63 Impact Factor